1. Field of the Invention
The present invention relates to a manufacturing method of a magnetic material used for a magnetic refrigeration material, a magnetostrictive material, and so on, and to the magnetic material applying the method.
2. Description of the Related Art
In recent years, as an environment-conscious refrigeration technique, an expectation for a magnetic refrigeration which is clean and has a high energy efficiency is increasing. On the other hand, as a magnetic material for the magnetic refrigeration, a substance in which a large magnetic entropy change can be obtained near a room temperature is found. As such a magnetic substance for the magnetic refrigeration, (Hf, Ta)Fe2, (Ti, Sc)Fe2, (Nb, Mo)Fe2, La(Fe, Si)13 having an NaZn13 type crystal structure, and so on are known.
Among these magnetic refrigeration substances, a substance represented by a chemical formula such as La(Fe, Si)13, having the NaZn13 type crystal structure is especially attracting attention. In such substance, Fe mainly enters into a position corresponding to Zn of a phase having the NaZn13 type crystal structure (hereinafter, referred to as NaZn13 crystal structure phase), and La mainly enters into a position corresponding to Na (hereinafter, this substance is abbreviated as LaFe13 based magnetic material). In the LaFe13 based magnetic material, the large magnetic entropy change can be obtained while a main constitutional element thereof is inexpensive Fe. Besides, it has a promising property as a practical magnetic refrigeration substance such that a temperature hysteresis does not occur in a magnetic phase transition (for example, refer to Japanese Patent Laid-open Application No. 2002-356748, and Japanese Patent Laid-open Application No. 2003-096547).
As a manufacturing method of the LaFe13 based magnetic material, it is reported that a magnetic material whose main phase is the NaZn13 crystal structure phase can be obtained by performing an integration of a raw material using an arc melting method and so on, and subsequently, by performing a heat treatment holding at 1000° C. for approximately a month (refer to X. X. Zhang et al., Appl. Phys. lett., Vol. 77, No. 19 (2000)). During a creating process of the LaFe13 based magnetic material, a lot of α-Fe phases are included at a stage when the integration (alloying) of the raw material is performed by applying the arc melting method or a high frequency melting method, and the NaZn13 crystal structure phase is rarely generated. Consequently, it is necessary to perform the heat treatment in high temperature and for a long time to obtain the LaFe13 based magnetic material from the integrated alloy.
On the other hand, a generation of the α-Fe phase being a stable phase is suppressed and the NaZn13 crystal structure phase is generated, by forcibly cooling a molten metal of the raw material composing the LaFe13 based magnetic material at a cooling speed of approximately 1×104° C./s to solid, instead of naturally cooling the molten metal to solid. Incidentally, it is generally known that the cooling speed of an alloy molten metal is at approximately 1×102° C./s in the melting method represented by the high frequency melting or the arc melting, but a cooling can be performed at a speed of 1×104°C/s or more in a liquid quenching method represented by a cooling using a single-roll equipment. Here, the cooling at the speed of 1×104° C./s or more is expressed as a forced cooling.
For example, a method in which an alloy is formed by quenching (forced cooling) a raw material molten metal being the LaFe13 based magnetic material whose main constituent is Fe, and a heat treatment is performed to this alloy at a temperature of 400° C. to 1200° C., is described in Japanese Patent Application Laid-open No. 2004-100043. A time for heat treatment can be reduced by applying such a method, but the main phase thereof is still the α-Fe phase even in an quenched alloy. Consequently, the heat treatment is inevitable to make the NaZn13 crystal structure phase as a main phase. Further, when a quenched material in a thin-band state or a spherical state is grinded to be used as a particulate magnetic refrigeration material, there is a problem that a uniformity of composition between particles is lowered because many α-Fe phases are contained. In addition, the more there are the α-Fe phases, the more it becomes difficult to grind.
In Japanese Patent Laid-open Application No. 2004-099928, it is described that the LaFe13 based magnetic material having the NaZn13 crystal structure phase can be obtained just after a casting, by containing boron (B), carbon (C), and so on within a raw material composition of the LaFe13 based magnetic material in the range of 1.8 atom percent to 5.4 atom percent. However, there is a problem that a compound phase containing B, for example, such as F2B phase exists as a hetero-phase in the alloy cast by this method, in accordance with an addition of B and so on to the raw material. A generation of the compound phase of Fe, B, and so on becomes a factor to deteriorate characteristics of the LaFe13 based magnetic material.
As stated above, in the manufacturing process of the LaFe13 based magnetic material useful as the magnetic refrigeration material and the magnetostrictive material, the heat treatment for a long time is required to obtain the NaZn13 crystal structure phase, and therefore, there is a problem that a productivity thereof is extremely low caused by this long time heat treatment. Further, an oxygen amount within the material becomes relatively large, and magnetic characteristics of the LaFe13 based magnetic material become easy to be lowered when the long time heat treatment is performed. It is difficult to completely eliminate the use of the heat treatment even when the NaZn13 crystal structure phase is preferentially generated by applying the forced cooling. In addition, the material obtained by the forced cooling is in the spherical state or in the thin-band state, and therefore, there is a problem that a flexibility in shape is low.